Hackers responsible for an ongoing series of attacks against US banks over the past week may be tapping into botnets to power their assaults, according to security researchers. Meanwhile, the Financial Services ISAC (Information Sharing and Analysis Center) continues to advise banks to be prepared for attack.

Bank of America, Citigroup, US Bancorp, JPMorgan Chase, Wells Fargo and PNC have all been hit by DDoS attacks for which hacker group the Izz ad-Din al-Qassam Cyber Fighters took credit via a series of posts to PasteBin: http://pastebin.com/u/QassamCyberFightersThe hacktivist group claimed its actions had been prompted by indignation over the Innocence of Muslims, an amateur anti-Islamic film whose trailer had appeared on YouTube. The same film has also provoked riots and attacks on Western diplomatic missions across the world.

The DDoS attacks have been responsible for intermittent disruption and slowdowns for bank customers trying to use the targeted websites.

Blasco said attackers "must have had help from other sources" such as someone with access to botnet networks of compromised PCs or contacts in a well-resourced group, such as a nation state. One of the members of infamous hacktivist crew LulzSec allegedly owned a botnet used in its attacks, providing a zombie attack precedent of sorts.

Independent US Senator Joe Lieberman told NBC during a television interview that he believed Iranian government sponsored hackers were involved in the attacks, suggesting the country's involvement was motivated by a desire to retaliate against Stuxnet and related attacks against Iran's nuclear programme. Gholam Reza Jalali, the head of Iran’s Passive Defense Organization, who is in charge of the country's cyber programme, has denied these claims, which remain unsubstantiated.

Hacktivist statements associated with the ongoing attacks on US banks demand the erasure of the Innocence of Muslims from the interwebs.

The Financial Services ISAC raised its cyber threat level to "high" on 19 September, around the time of the first attacks targeting Bank of America, and the net attack risk outlook remains at the organisation's second-highest state of alert:http://www.fsisac.com/

Dan Holden, director of research at Arbor's Security Engineering and Response Team (ASERT), said the attacks were almost certainly powered by botnets.

"This attack has generated enormous amount of attention because it was telegraphed in advance, putting these firms on notice," Holden told El Reg. "That has led to great media interest and frankly, some pretty wild speculation about motives and attack techniques.

"Arbor does not believe that this was an opt-in only attack, but one driven heavily by botnets as well. This incident shows the need for businesses to take a proactive approach to the issue of network availability. Trying to fix your roof when it's raining is not pleasant experience."